The present invention relates to a drivetrain for a motor vehicle, having a friction clutch for transmitting drive torque for the motor vehicle and having an actuator arrangement for actuating the friction clutch.
The use of friction clutches in drivetrains has long been known. For many years, dry friction clutches have been used in conventional manual shift transmissions as starting and separating clutches, to name just one example.
For a number of years, there has also been a trend towards automatic drivetrains in which friction clutches are opened and closed automatically. This relates on the one hand to the starting and separating clutches of automatic shift transmissions. The so-called dual-clutch transmissions generally use wet-running multi-disc clutches for the two separating clutches.
Friction clutches are also increasingly used in all-wheel-drive systems. In vehicles with all-wheel-drive, a distinction is made between clutch-controlled torque distribution and differential-controlled torque distribution. In clutch-controlled systems, one axle is generally driven at all times, and the second axle can be connected. Here, friction clutches are used, for example in the so-called “Haldex system” in which the second axle is connected on demand by means of a multi-disc friction clutch of said type. In differential-controlled systems, a friction clutch can, as a differential lock, be connected in parallel to the differential which provides a fixed-ratio torque distribution between the two driven axles. Friction clutches such as multi-disc friction clutches are also used here, since locks of said type can also be automatically actuated while travelling and under load in order to improve propulsion.
All-wheel-drive locks of said type, which can be activated or deactivated independently of differential speeds at any time, are also called “active” locks. In vehicles which are regularly also used in normal road traffic, it is important that such lock systems are compatible with safety systems such as ABS and ESP. Since a closed differential lock greatly influences the driving behaviour, a lock of said type must be capable of opening or releasing quickly. In addition, a differential lock of said type should be open in the non-actuated state (“fail safe”).
The actuator arrangements for friction clutches of said type comprise, on the one hand, solutions based on an electric motor. Here, the disc stack is actuated by means of a ball ramp which is in turn actuated electromotively via a small gear mechanism. The application force is provided by means of the mechanical step-up ratio (that is to say the toothing of the gear mechanism and the ball ramp).
Alternatively, all-wheel-drive clutches of said type can be hydraulically actuated. Here, it is not unusual to provide, in the hydraulic circuit, an electrically driven pump and a pressure accumulator for maintaining a high actuating pressure. Such additions often entail a high expenditure on valves and other hydraulic components in order to be able to meet the requirements stated in the introduction.
It is not unusual nowadays for friction clutches for all-wheel-drive vehicles to be operated for relatively long periods in the closed state, for example in off-road operation. Here, active periods of one hour are not uncommon. In the above-described solution based on an electric motor in connection with a mechanism and a ball ramp, this leads to the electric motor heating up, since a very high current consumption (in the region of 30 A and above) is required to keep the clutch in the closed state. As a result of the heating effect, the actuator arrangement has a low degree of efficiency. The same problem occurs if a friction clutch is actuated directly by means of a pump.